Why Retirements May Hold the Key in Whether Republicans Can Keep the House

Democrats often lament that the House electoral playing field is stacked against them, but that’s not their only problem.

The Republican structural edge in the House is fully realized only with the added advantage of incumbency. The Democrats would be overwhelming favorites to retake the House in this political environment if no incumbents chose to run for re-election, even with all of the burdens of gerrymandering and geography.

So far, four Republicans have announced they will not seek re-election in highly competitive districts (with a Cook Partisan Voter Index of Republican+5 or more Democratic; the Cook P.V.I. measures district partisanship based on its vote in recent presidential elections).

My colleagues Jonathan Martin and Alex Burns reported recently that Republicans fear a wave of additional retirements. And Roy Moore’s victory over the establishment-backed Luther Strange in the Alabama Republican Senate primary this week could further embolden primary challenges to G.O.P. House incumbents, increasing the chances that they step aside.

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Charlie Dent, a Republican representative from Pennsylvania, has said he will not seek re-election in 2018. So far, four Republicans have retired in highly competitive House districts.CreditJ. Scott Applewhite/Associated Press

But without a string of retirements, the Democratic path to control of the House is challenging. Incumbents are just that hard to beat.

Democrats often look at the districts carried by Hillary Clinton in the 2016 presidential election as top-tier targets. But it won’t be so easy. Based on recent elections, Democrats would do well to defeat even half of the incumbents running for re-election in those districts.

Over the last decade or so, incumbents have run about seven percentage points ahead of non-incumbents from the same party in similar districts. That’s more than enough to let incumbents in competitive districts survive, even in so-called wave elections like the ones that swept Democrats into power in the House in 2006 and back out in 2010.

Here’s a different way to think about it. In wave elections like those in 2006, 2008 and 2010, a majority of incumbents from evenly divided districts (based on Cook P.V.I.) managed to win re-election. Even a large number of incumbents in districts that tilted heavily toward the other party managed to win re-election.

Of course, the party in control of Congress always has incumbency on its side. But, as Democrats accurately lament, the party out of power has never had such a limited set of options to defeat the other side’s incumbents.

There are just nine districts that tilt toward the Democrats by even the slightest margin according to the measure of Cook P.V.I., well short of the 24 needed for regaining control of the House. There were — get this — 76 Democratic-held, Republican-tilting (or often leaning) districts by that measure heading into the 2010 election. There were 24 Republican-held, Democratic-tilting districts in 2006. So there are vastly fewer top-tier opportunities for the Democrats to pick off Republican incumbents.

Without a surge of Republican retirements, a Democratic House takeover would probably require the party to do even better at defeating Republican incumbents than it did in 2006 or the Republicans did in 2010. If Democrats merely matched those results, they would probably fall a bit short of the 24 seats they need.

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Dave Reichert, a Republican member of Congress from Washington State, has announced he is retiring after seven terms.CreditJ. Scott Applewhite/Associated Press

This is not a prediction. The Democrats really could do even better at defeating incumbents than they did in 2006 or the Republicans did in 2010. The advantage of incumbency has been declining steadily over the last few decades, and it has fallen to around five points in the House elections since 2010. Even a somewhat smaller incumbency bonus could easily yield more victories for Democrats in 2018, perhaps especially in the highly competitive districts that supported Mrs. Clinton or Barack Obama. What this does illustrate, however, is that it’s an open question whether a wavelike political environment would be enough for the Democrats to take over the House.

A flurry of Republican retirements could change that quickly.

Despite Republican-led gerrymanders and the Democratic clustering in urban areas that tends to waste votes, Mr. Obama won around 240 of today’s congressional districts while winning the national popular vote by seven points in 2008. That’s a rough but reasonable approximation of how many seats the Democrats might win in a wave election if no Republican incumbents ran for re-election, not the 213 seats they might win by doing as well against incumbents as the party out of power did in 2006 and 2010.

The more Republicans who choose not to run for re-election, the more the expected Democratic tally moves toward 240.

Since 2004 (and excluding 2012, because of redistricting), around 15 percent of the president’s party’s incumbent representatives in potentially competitive districts have retired in an election cycle on average. This time, that would translate to seven retirements — or three more than the four so far. Moving 15 percent of the way from 213 to 240 would be enough to make the fight for control of the House a true tossup, as prediction markets and most analysts seem to expect.

Democrats could become clear favorites in a wave election if retirements went well beyond typical levels. The most favorable precedent for Democrats is 2008, when Republicans were newly out of power in the House and poised for another general election defeat. In that cycle, 27 percent of Republicans in competitive districts chose not to run for re-election. This year, that would mean an additional nine retirements in competitive districts.

It seems hard to imagine another exodus on that scale. Republicans still control the House and their caucus is quite young, with only a handful of vulnerable Republicans over age 65. But so far, there have been more Republican retirements than there were at this point in 2008 or any other cycle since 2004, according to an analysis from Daniel Donner of DailyKos.

If retirements were to stay ahead of 2008 pace, it would be enough to make Democrats favorites to win the House in a wave election like those in 2006 or 2010. They still wouldn’t necessarily win as many seats as the winning party did in those elections, because of geography and gerrymandering. But if they did as well as the typical party out of power in the past, it would probably get them over the top.

Correction:

An earlier version of this article misstated the Michigan congressional district that Dave Trott represents. It is the 11th, not the Eighth.

A version of this article appears in print on , on Page A17 of the New York edition with the headline: Republicans’ Hold on the House May Hinge on Retirements. Order Reprints | Today’s Paper | Subscribe